[Feature] Add option to set `Authorization` header

[eiabea]

Problem

It is currently not possible to send additional headers in the wallet for custom networks. This prevents us from using our own RPC endpoints due to the fact, that these endpoints are secured by an api-key middleware.

Proposal

• Add additional field "API Key" to Advanced settings - Manage networks - <Network name> (probably below "RPC URL"
• Use this "API Key" value to set the Authorization header in the rpc requests (Authorization: Bearer <API Key value>)